1. Field of the Invention
The invention relates to the field of energy beam imaging, and in particular, to a method and apparatus for x-ray imaging sub-volumes by sequentially filtering and focusing individual source points with a confocal coded aperture.
2. Description of Related Art
The ability to non-destructively analyze microscopic volumes within large objects, for example multi-layer microelectronic packages, requires use of penetrating radiations such as X-rays. Current techniques for inspecting these volumes typically employ transmission radiography, which is similar to medical X-ray imaging. Computed tomography (CT) can be used to view individual planes of interest in such packages. The resolution of these systems is limited by the x-ray source, that is, the system resolution defined by the spot size of the X-ray source. Transmission methods suffer from an inability to differentiate three-dimensional information. In other words, a p three-dimensional space must be mapped onto a two-dimensional plane. Moreover, techniques such as CT require that the whole object be inspected, which can unnecessarily generate large quantities of data and require computationally intensive and time-consuming reconstruction processes to mitigate CT image artifacts.
Coded aperture imaging systems were historically developed to image points of non-focusable radiation from sources such as x-ray or gamma stars, and discrete, localized nuclear events. The literature describes many attempts to use coded apertures to image extended two-dimensional and three-dimensional objects, but the results have never exceeded the type of resolution and detail achievable through other means, such as transmission or computed tomographic (CT) radiography. This limitation on resolution and detail is primarily due to the coded aperture impulse or frequency response and the response of the system to out-of-plane sources of scattered radiation. Since all previous systems attempt to image the entire object instantaneously, the degrading effects of the aperture quickly become dominant as the size of the object increases.
The technology associated with coded aperture imaging has been around since the late 1970's. The earliest work in the field, using coded aperture arrays, is described in "Coded Aperture Imaging with Uniformly Redundant Arrays", E. E. Fenimore, and T. M. Cannon, Feb. 1, 1978, Applied Optics, Vol. 17, No. 3, pp. 7-347.
Coded aperture imaging using neutron sources during the early 1990's is described in "Three-Dimensional Information from Real-Time Encoded Images", K. W. Tobin, J. S. Brenizer, and J. N. Mait, Optical Engineering, Vol. 29, No. 1, January, 1990.
The nature and characteristics of Fresnel zone plate (FZP) X-ray source focusing technology is described in "Design and Fabrication of Fresnel Zone Plates with Large Number of Zones", Z. Chen, Vladimirsky, M. Brown, Q. Leonard, O. Vladimirsky, F. Moore, F. Cerrina, B. Lai, W. Yuri, and E. Gluskin, J. Vac., Sci. Technology, B 15(6), November/December 1997, p. 2522.
An x-ray spectrometer is described in U.S. Pat. No. 5,757,005--Callas, et al. that provides images of an x-ray source. Coded aperture imaging techniques are used to provide high resolution images. Imaging position-sensitive x-ray sensors with good energy resolution are utilized to provide what is described as excellent spectroscopic performance. The system produces high resolution spectral images of the x-ray source which can be viewed in any one of a number of specific energy bands.
A system utilizing uniformly redundant arrays to image non-focusable radiation is disclosed in U.S. Pat. No. 4,209,780--Fenimore, et al. A uniformly redundant array is used in conjunction with a balanced correlation technique to provide a system said to have no artifacts, such that virtually limitless signal-to-noise ratio is obtained with high transmission characteristics. Additionally, the array is formed as a mosaic to reduce required detector size over conventional array detectors.
Many other patents and publications reference coded aperture imaging techniques but they are all based on imaging a single, localized source of scattered energy, such as celestial sources, or instantaneously imaging a distributed source of energy from an extended body. There are no other known techniques that utilize the inventive arrangements taught herein.
Accordingly, there is a need for a new method and apparatus for imaging three-dimensional objects, particularly for imaging small volumes of interest within a larger volume or object, using non-focusable radiation.